Giant alpine spider (English)

Vesubia jugorum is a large-sized spider (body length: 15–20 mm, prosoma: 7-9 mm). The prosoma is generally blackish or dark brown, marked with black streaks irradiating from the fovea. The opisthosoma is dark grey dorsally and brown reddish ventrally. Legs are dark brown dorsally and reddish-yellowish ventrally, especially on coxae (Figs 1, 2​). See Tongiorgi (1968), Tongiorgi (1969), Maurer and Thaler (1988) and Nentwig et al. (2016) for genitalic drawings and other relevant diagnostic features.

Figure 1.

Habitus of Vesubia jugorum — female. [Photo credit: Emanuele Biggi, 2003]

Figure 2.

Vesubia jugorum — female with spiderlings. [Photo credit: Emanuele Biggi, 2016]

We based the Species Distribution Model (SDM) on literature data (Isaia et al. (2015), Isaia et al. (2007), Maurer and Thaler (1988), Simon (1881), Tongiorgi (1968), Tongiorgi (1969)) and original unpublished data gathered during recent surveys (see New occurrences).

Occurrences were used to model the current distribution of the species through a MaxEnt model in dismo R package (Hijmans et al. 2014). See Mammola et al. (2015) and Isaia et al. (2016) for details on modeling procedure. We estimated the extent of occurrence (EOO) and the area of occupancy (AOO) from the model, as implemented in the red R package (Cardoso 2016).

To estimate the potential variation of the EOO and AOO due to future climate change, the model was projected in the future (year 2028; i.e. 3 spider generations) according to two different representative concentration pathways, namely rcp 2.6 (low emission rate) and rcp 8.5 (high emission rate).

Vesubia jugorum was originally described from an unspecified locality at high altitude in the vicinity of St. Martin-Vésubie (Haute Vésubie Valley, France). The range of this stenoendemic species is centered on the Maritime Alps (43 records). Two additional subpopulations occur at the eastern and north-western corners of the range, in Ligurian (2 records) and Cottian Alps (2 records). Most localities are situated in the Site of Community Importance and Special Area of Conservation IT1160056 “Alpi Marittime” (NW Italy).

The species inhabits rocky areas of the subnival and nival zones of the Maritime Alps. The high altitude regions are particularly vulnerable to climatic variations due to climate change, with warming rates approximately doubling the global average (Böhm et al. 2001). In consequence of temperature increase, range shifts towards higher latitudes or altitudes are expected (Bellard et al. 2012), causing a decrease in the EOO. In particular, we predict a future reduction of the EOO ranging from 24% (low emission scenario) to 41% (high emission scenario).

Future forecasts based on different emission scenarios show a significant reduction in the bioclimatic range of Vesubia jugorum (details in Isaia et al. (2016)). We predict a future reduction of the AOO ranging from 12% (low emission scenario) to 32% (high emission scenario). In this perspective, it is important to take into account the connectivity of the suitable habitat, in order to provide coherent interpretations of the future trends. Suitable ecological corridors for this species would be represented by high-altitude rocky areas, which rarely occur between the Maritime Alps and the northern Alpine districts. The species is probably also limited by its low dispersal ability.

The whole population is affected by the ongoing climate change.

Inferred from the decline in AOO.

The main subpopulation of the species is centred in Maritime Alps. This core area includes over 90% of the known localities. Two additional subpopulations are identified at the north-western and eastern corners of the distribution range, corresponding to the Southern Cottian and Ligurian Alps, respectively. All future warming scenarios predict the extinction of the latter subpopulation.

Within the core area the suitable habitat is roughly continuous, with peaks and rocky areas ensuring connectivity between local populations. However, unsuitable habitat —namely grasslands at lower altitudes— reduces connectivity between subpopulations.

The species is restricted to alpine rocky areas above 2,000 m. These include rocky debris, boulder fields, and alpine screes (Figs 4, 5).

Figure 4.

Typical rocky areas inhabited by Vesubia jugorum — Maritime Alps, Province of Cuneo. [Photo credit: Emanuele Biggi, 2016]

Figure 5.

An example of rocky areas colonized by Vesubia jugorum — Maritime Alps, Province of Cuneo. [Photo credit: Filippo Milano, 2016]

The optimal habitat for the species is not expected to undergo significant variations in the future, as touristic pressure is negligible in the high mountain peaks of the Maritime Alps. Altitudinal shifts of vegetation due to climate change may hypothetically affect the extension of the subnival zone determining small variations in the extent of high altitude rocky areas.

The species is active during the warm season, from the snow melting in late June to mid September. Males are mainly found from July to August, whereas females and immatures can be found throughout the whole season. During the day, individuals can be observed wandering on the rocks. Females with cocoons build retreats under stones (see illustration in Tongiorgi (1969)), usually of 10–15 x 3cm. Preliminary data on reared individuals and estimations based on body size suggest generation length > 3–4 years.

Most of the species range falls within the borders of national parks, sites of community importance and special protection areas, namely Parco Naturale Alpi Marittime (Italy), Parco Naturale del Marguareis (Italy) and Parc National du Mercantour (France).

In the light of the existing threats, it is expected that the survival of the species will depend on monitoring, conservation management and translocation programmes. Ex-situ conservation should also be considered, to ensure the preservation of healthy individuals for re-introduction in suitable habitats situated to the north from the current distribution area.